Chemical vapor detection systems relying on point-sensors are of practical importance because of their potential for excellent performance with small size, low power, and low cost, a combination that is highly desirable for various hand-held and autonomous applications.
One main class of such systems is chemielectric in nature and functions by incorporating an electrically contacted transduction layer that undergoes a change in some electrical property (e.g., resistance or capacitance) in response to the sorption and desorption of the chemical vapor. Examples of materials used in this role include films formed of monolayer-encapsulated gold nanoclusters (metal-insulator-metal ensemble, or MIME), single-walled carbon nanotubes (CNT), graphene, and reduced graphene oxide (rGO), and the recently developed trilayer films of transition metal dichalcogenides (TMD). See Arthur W. Snow, F. Keith Perkins, Mario G. Ancona, Jeremy T. Robinson, Eric S. Snow, and Edward E. Foos, “Disordered Nanomaterials for Chemielectric Vapor Sensing: A Review,” IEEE Sensors Journal, Vol. 15, No. 3, March 2015, pp. 1301-1320.
The temperature-dependent desorption of a vapor from a solid surface in vacuum is a phenomenon used by an analytical technique known as thermal desorption spectroscopy (TDS) or temperature programmed desorption (TPD) to investigate surface chemical interactions. See P. A. Redhead, “Thermal desorption of gases,” Vacuum 12, 203 (1962). For analysis purposes, TDS is generally used on carefully prepared homogeneous surfaces and is done in ultra-high vacuum so that no adsorption or re-adsorption occurs during the measurement and mass spectrometry may be used.
In this technique, a mass spectrometer is used to monitor desorption of molecules from an initially very cold substrate as its temperature is raised, with inferences regarding the detailed chemical interactions at the surface being drawn from the results. Measurements using this technique are typically taken at temperatures well below room temperature.
FIG. 1A shows the results of measurements taken using this technique for detection of various organic compounds on carbon nanotubes, while FIG. 1B shows the results of measurements (in arbitrary units) taken for desorption rate of deuterated water from two organic functional group surfaces (—COOH and —CH3) after different levels of exposure to water vapor. The table shows calculated desorption energies Ed for water from surfaces comprising the listed chemical functional groups. See J. Goering, E. Kadossov, and U. Burghaus, “Adsorption kinetics of alcohols on single-wall carbon nanotubes: An ultrahigh vacuum surface chemistry study,” J. Phys. Chem. C 112, 10114 (2008) (carbon nanotubes); and R. L. Grimm, N. M. Ballentine, C. Knox, and J. C. Hemminger, “D2O water interaction with mixed alkane thiol monolayers of tuned hydrophobic and hydrophilic character,” J. Phys. Chem. C 112, 890 (2008) (water desorption v. temperature, —CH3 and —COOH surfaces) and L. H. Dubois, B. R. Zegarski, and R. G. Nuzzo, “Fundamental studies of microscopic wetting on organic surfaces. 2. Interaction of secondary adsorbates with chemically textured organic monolayers,” J. Am. Chem. Soc. 112 570 (1990) (organic monolayers).
Another approach to chemical vapor point-sensor systems has been developed by Stephen Semancik and his group at the National Institute of Standards and Technology (NIST) in the area of metal oxide sensors. These type of sensors are widely used, for example, the sensors available commercially from Figaro USA, Inc., and rely on a modulation of the grain boundary electrical resistance of materials like SnO2 or ZrO2 that occurs upon exposure to gas vapors (O2, COx, NOx) at high temperature (500-800° C.). They also induce the oxidative decomposition of the detected vapor. The original innovation of the NIST group was to combine the metal oxide sensors with microhotplates in order to greatly reduce the electrical power needed to provide the high temperatures. See J. S. Suehle, R. E. Cavicchi, M. Gaitan, and S. Semancik, “Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing,” IEEE Elect. Dev. Lett. 14, 118 (1993). Since then he and his group have exploited the temperature control offered by the microhotplate plus computer learning algorithms to enhance both selectivity and response time. See B. Raman, R. Shenoy, D. C. Meier, K. D. Benkstein, C. Mungle, and S. Semancik, “Detecting and recognizing chemical targets in untrained backgrounds with temperature programmed sensors,” IEEE Sensors J. 12, 3238 (2012); and A. Vergara, K. D. Benkstein, C. B. Montgomery, and S. Semancik, “Demonstration of fast and accurate discrimination and quantification of chemically similar species utilizing a single cross-selective chemiresistor,” Anal. Chem. 86, 6753 (2014).
A further approach is that of Thomas Thundat and his co-workers at Oak Ridge National Laboratories, who use microfabricated bridges to detect explosive molecules by calorimetry. See L. R. Senesac, D. Yi, A. Greve, J. H. Hales, Z. J. Davis, D. M. Nicholson, A. Boisen, and T. Thundat, “Micro-differential thermal analysis detection of adsorbed explosive molecules using microfabricated bridges,” Rev. Sci. Instr. 80, 035102 (2009).
Although the “dream” for such sensors is to achieve a dog-like olfactory acuity and form factor, their actual performance is very limited, especially with regard to selectivity, i.e., their ability to discriminate between the target vapor(s) and other atmospheric constituents. For this reason, except for a few analyte-specific implementations such as the conjugated polymer-based chemical sensors produced by Swager and co-workers or the FIDO® chemical sensors produced by FLIR Systems, Inc., chemielectric sensors are never used alone, but instead are almost always combined in an array format with each sensor in the array having a different (and hopefully “orthogonal”) response spectrum. The idea is then to use software (often referred to as “chemometrics”) to combine the outputs and improve selectivity, even in an environment cluttered with multiple vapors. See J. W. Grate and B. M. Wise, “A Method for Chemometric Classification of Unknown Vapors from the Responses of an Array of Volume-Transducing Sensors,” Anal. Chem. 2001, 73, 2239-2244.
A second important selectivity strategy often implemented in conventional point-detection systems for chemical vapors is a micro-gas-chromatograph (μGC), together with a pump and a standard-air supply in order to provide the needed discrimination. See S. Zampolli, I. Elmi, F. Mancarella, P. Betti, E. Dalcanale, G. C. Cardinali, and M. Severi, “Real-time monitoring of sub-ppb concentrations of aromatic volatiles with a MEMS-enabled miniaturized gas-chromatograph,” Sensors and Actuators B 141 (2009) 322-328.
For many vapors of interest, not only is selectivity an issue, but so is the insufficiency of chemielectric sensor sensitivity. To enhance performance in this regard, a preconcentrator is often added to the system, where the preconcentrator collects vapor over an extended period of time and then releases it (via a thermal pulse) abruptly onto the μGC/sensor. See I. Voiculescu, M. Zaghloul, and N. Narasimhan, “Microfabricated chemical preconcentrators for gas-phase microanalytical detection systems,” Trends in Analytical Chemistry, Vol. 27, No. 4, 2008, pp. 327-343.
Given all of these additions needed to improve performance, a conventional chemical vapor sensor apparatus will typically include the chemielectric sensor itself plus a source of scrubbed air, an air sampling device, a preconcentrator, a micro gas chromatograph, and a micropump. Each of these additions adds to the system complexity, size, power, and cost. This tradeoff is unfortunate in that the improvements come only through degrading the very qualities that are supposed to recommend a point-sensor approach.
An important operational issue regarding the conventional chemielectric point sensor approach is that it invariably involves taking a difference between two measurements, one taken prior to vapor exposure (or often upon exposure to scrubbed air) and the other during vapor exposure. A critical implication of this fact is that the measurement time for conventional chemielectric point sensor systems is set by sorption-desorption kinetics, by the sensor enclosure dead volume, and by fluid flow times that are on the order of seconds or larger. This forces the operation to be quasi-dc (i.e., very low frequency) and that implies that the relevant noise floor is electrical 1/f noise and/or drift which tends to be large (and always much bigger than “chemical noise”). And since the sensitivity is set by a signal-to-noise ratio, it is the 1/f noise floor that ultimately limits the sensitivity of the conventional systems. See Snow et al., supra; see also W. Kruppa, M. G. Ancona, R. W. Rendell, A. W. Snow, E. E. Foos, and R. Bass, “Electrical noise in gold nanocluster sensors,” Applied Physics Letters 88, 053120 (2006).